In a fuel injection system of an internal combustion engine, a three-way valve is provided which is able to selectively connect a back pressure control chamber formed on an inside end face of a needle valve and an intermediate chamber of a booster piston for increasing an injection pressure to a high pressure fuel feed passage or low pressure fuel return passage. A fuel injection system designed to use the fuel passage switching action of this three-way valve for control for opening and closing a needle valve and for control for increasing the injection pressure by the booster piston is known (for example, see Japanese Patent Publication (A) No. 2003-106235). In this fuel injection system, the fuel passage switching operation by the three-way valve enables the phase difference between the opening timing of the needle valve and the start timing of the boosting action by the booster piston to be changed and thereby enables the injection rate of the fuel to be controlled to a desirable injection rate for the engine operating state.
However, in this fuel injection system, at the time of the fuel passage switching action by the three-way valve, the high pressure fuel feed passage ends up being connected with the low pressure fuel return passage. As a result, the problem arises of a large amount of high pressure fuel in the high pressure fuel feed passage ending up leaking into the low pressure fuel return passage. Further, if a large amount of high pressure fuel ends up leaking in this way, the problem also arises of the high pressure fuel pump feeding the high pressure fuel becoming insufficient in capacity.